i would either just go to a salon and tell them what you want to leave alone (i would almost say leave the middle section dark and make the top and underside the blonde : P)

or try a color wash at home and try to not get it on your blonde too much. if you get the blonde one it just might make it a more golden blonde but it will help lighten the rest

i used this http://www.becomegorgeous.com/redirect.php?url_red=http://www.productwiki.com/john-frieda-brilliant-brunette-luminous-color-glaze/" target="_blank">http://www.productwiki.com/john-frieda-brilliant-brunette-luminous-color-glaze/

and it worked to help lighten my hair to even out when an old stylest dyed the bottom part of all my hair almost black (she was trying to cover the red tha thad almost completely grown out)

and it worked great

or you can duy the top at home yourself get your mom to help and you need to either put vasoline on your bottom color to MAKE SURE you don't color it then roll it up in tin foil like you were doing a foil or just do the foil part w/o the vasoline (vasoline will just be a dye blocker so the hair can't absorbe the color)

then put your blonde color on the top like normal

be careful if you dyed it recently it might turn out funny if you try to re dye it but the wash would be a safe one